1. Field of the Invention
The present invention generally relates to the field of rubber recycling and more particularly, the invention pertains to producing devulcanized or reclaimed rubber from waste vulcanized rubber.
2. Description of the Related Art
Rubber products, such as automotive tires, are usually manufactured by processing (molding, extruding, calandering) and subsequent vulcanization of raw rubber materials. Generally, vulcanization is the chemical treatment of a rubber polymer molecules by cross-linking agent such as sulfur. The addition of sulfur and other special components provides desired physical properties of a final rubber product, such as strength, elasticity, durability etc. Vulcanized rubber is the source of one of the most serious waste product problems. The benefits achieved by vulcanization present a problem when attempting to recycle vulcanized rubber.
One of the common known approaches to recycling rubber waste is devulcanization that is intended to depolymerize rubber molecules or break the polysulfide linkages. Devulcanization techniques may include mechanical shear, high-energy radiation, chemical processing, thermo-mechanical processing, etc. In chemical terms, devulcanization means reverting rubber from its thermoset, elastic state back into a plastic, moldable state. This is accomplished by severing the sulfur bonds in the molecular structure. With the proper devulcanization method, a much higher percentage of waste rubber, including old tires can be re-used.
1. It has long been known that mere fine grinding, without any additives, brings about cleavage of sulfur bonds and formation of free radicals. Some early reclaiming processes e.g. disclosed in DE Pat. No. 4425049 used fine grinding at roller mill for this purpose. Later group of American and Japanese scientists found that this process occurs effectively in biaxial extruder at high temperatures that are disclosed in the following patents and patent application: U.S. Pat. No. 6,576,680; U.S. Pat. No. 6,590,042; U.S. Pat. Appl. No. US 2001025060. However, it was proved that such process is limited by adverse reaction of so-called “creeping re-vulcanization”, see B. Adhikari, D. De et al. Reclamation and recycling of waste rubber. Progress in Polymer Science. 25, 2000. 909-948 and PCT patent application No. WO0129122. From the above reasoning it is clear that mechanical destruction only is not capable to provide y reclaimed rubber materials of appropriate quality.
Some traditional devulcanization methods use exposing cured rubber to elevated temperatures for an extended period of time, e.g. by applying superheated steam as disclosed in PCT patent application No. WO9920380. Also is known applying alkali and softeners as further disclosed in U.S. Pat. No. 4,161,464 and U.S. Pat. No. 5,798,394. However, this “digesting reclaim process” not only severs the sulfur bonds in the polymer matrix, but also breaks the polymer chains, causing a significant degradation of physical properties of the rubber. Due to questionable cost efficiency and environmental problems, thermal devulcanization is rarely used today.
Another technique uses prior swelling of cured material by solvents (mainly, butanol) in order to facilitate subsequent destruction of sulfide bonds as disclosed in U.S. Pat. Appl. No. US20020091167 and No. US20030225171. That technique requires removal of solvent and drying reclaimed material making entire process impractical.
Reclaimed materials based on rubber crumb with binders are also widely used for manufacturing simple articles (tiles, carpets, floors etc.). Usage of various binders in form of adhesives (such as polyurethane) and polymer additives (such as PP, EVA etc.) are disclosed in the following patents and patent application U.S. Pat. No. 5,397,825; PCT Pat. Appl. No. WO9948960; U.S. Pat. No. 5,303,661 and U.S. Pat. No. 4,378,067. Such reclaimed materials have limited usage due to poor physical properties thereof.
Various oxidizers for stabilization free radicals, formed in grinding process, have drawn attention of rubber chemists. Chlorine as oxidizing agent is used in technologies disclosed in U.S. Pat. No. 5,693,714; U.S. Pat. No. 5,506,283 and U.S. Pat. No. 5,438,078. Due to poor quality of reclaimed material and polluting effect of chlorination agent, this direction did not gain acceptance among industrialists.
Better physical properties of devulcanizate could be obtained by applying ozone as oxidizing agent as disclosed in LV patent No. LV13339B of Zagars. Unfortunately, grinding under ozone stream requires too complicated equipment, making this technology impracticable.
According to the following patents: U.S. Pat. No. 5,677,354; U.S. Pat. No. 5,798,394 and U.S. Pat. No. 5,891,926 devulcanization could be performed by applying some biotechnological reactions. These methods require complicated equipment, large floor space and excessive operating time.
Initially it was considered that cleavage of sulfide bonds occurs only at elevated temperature, but later it has been found that in the presence of amines and disulfides the devulcanization process accelerates sharply and even could be preformed at ambient temperature (see Krebs. Z. Anorg. Allg. Chemie. 276, 1954). This discovery has initiated a number of devulcanization technologies. One of them is represented in the following patent and patent applications: EP Pat. Appl. No. EP0690091 and U.S. Pat. No. 5,770,632 (both relate to so-called De-Link process). Other versions are disclosed in PCT Pat. Appl. No. WO2007062611 and WO0129121 and U.S. Pat. No. 6,924,319. The process gained wide acceptance due to simplicity of used standard equipment (roller mills). However, it was shown that sufficient disadvantages inhere to this technique. It requires fine grinding the cured rubber (up to 0.4 mm and less). Moreover, this method does not provide good selective action to different bonds and along with the attack of C—S and S—S bonds, it attacks also C—C bonds and initiates their deep destruction. This causes reduction of the physical and mechanical properties of secondary vulcanizate. According to these publications, secondary devulcanizate from tire scraps retains 62-70% of tensile strength as compared with primary vulcanizate.
Therefore, a need still exists for a method of devulcanization and chemical composition that provide efficient and simple devulcanization technique.